1. Field of the Invention
The present invention relates to a method and an apparatus for characterizing a recess located on a surface of a substrate.
2. Description of the Related Art
Currently, electronics is dominated by microelectronic semiconductor components with integrated circuits. Such integrated circuits include a complex arrangement of electronic structures which are disposed and interconnected with each other on tiny semiconductor substrates, also called chips. The combined production of integrated circuits on a semiconductor disc, also referred to as wafer, is characterized by a large number of successive process steps.
One main demand of the semiconductor industry is the continuous power enhancement provided by increasingly faster integrated circuits which is interrelated to a miniaturization of the electronic structures. In the course of this development, the semiconductor industry has changed over to producing increasingly three-dimensional structures with minimal lateral dimensions on the semiconductor wafers which require less space on the surfaces of the semiconductor chips. In general, recesses or vias located on the surfaces of the wafers serve as initial structures for the three-dimensional structures. Such recesses are typically produced by subjecting the wafers to specific photolithographic and etch processes.
The miniaturization of the electronic structures correlates to an increasing accuracy of the production processes. At the same time, accurate and reliable inspection procedures are required in order to control the production processes. With regard to three-dimensional structures, methods and devices for characterizing recesses located on the surfaces of the wafers have a great significance, as these methods make it possible to find out unwanted deviations from structure dimensions or defects, which may be due to incorrect production processes and which may affect the operability of produced semiconductor chips.
One of the currently used methods for characterizing recesses located on a surface of a wafer is based on the usage of a scanning electron microscope (SEM). For this method, the respective wafer is broken in the area of the recesses of interest and positioned in a vacuum chamber. Afterwards, the waste edge of the wafer is hit or scanned by a focused electron beam such that secondary electrons are knocked loose and emitted from the waste edge. The secondary electrons are collected by a detector in order to build up an image of the waste edge. Thus, for example, a depth of a recess may be characterized or an unwanted insulating or residue layer located at the bottom of a recess due to an incomplete etch process may be detected.
A drawback of this method is that the wafer is destroyed by the breakage. As a consequence, the method is very costly. Moreover, the method may only be applied offline to a few wafers and in particular, not to the product wafers in the production line. Accordingly, the results of the offline measurement may not be necessarily representative of measurements on product wafers.
In an alternative method for characterizing recesses located on the surface of a wafer, an atomic force microscope (AFM) is used. An atomic force microscope operates by scanning the surface of interest with a microscope tip which is fixed to a cantilever. By measuring attractive or repulsive interatomic forces between the tip and the surface of the wafer being inspected, topographical information on the surface and thus information on the lateral dimensions and the depth of a recess may be derived.
In order to decipher the depth of a recess, the microscope tip has to be sufficiently small so as to traverse the bottom of the recess. Due to the shrinking lateral dimensions of the structures and thus of the recesses, however, the requirements for the geometry, the stability and the abrasion resistance of the tip increase. In particular, in the case of recesses with a high aspect ratio, i.e., a high ratio between the depth and the lateral dimensions, the geometry of the tip and its manufacturability become increasingly critical. As a consequence, recesses with a high aspect ratio may not be sufficiently characterized by using an atomic force microscope.